1. Field of the Invention
The present invention relates in general to a apparatus for measuring the flow rate of intake air in an intake air passage of an internal combustion engine, which apparatus includes a heat-generating resistor element in the form of a hot wire or a hot film, for example, and more particularly to such an measuring apparatus wherein the heat-generating resistor element is effectively protected against dust in the intake air, and/or water, oil and other contaminants in the back fire from the engine.
2. Discussion of the Prior Art
As known in the art, various types of air flow meters are disposed in an intake air passage of an internal combustion engine, to detect the flow rate of intake air supplied to the engine. As one type of such air flow meters, there is known a thermal flowmeter as disclosed in JP-B2-1-45009, which includes a heat-generating resistor element in the form of a hot wire, hot film and the like, which is disposed in an intake air passage. The disclosed thermal flowmeter is adapted to determine the flow rate of the intake air, by detecting a change in the electrical resistance of the resistor element, which varies with the amount of heat transfer from the element to the air flowing through the passage.
In the thermal flowmeter thus installed in the intake air passage of the internal combustion engine, the surface of the heat-generating resistor element may be contaminated by dust contained in the intake air, or water, oil and others contained in the backfire blowing back from the engine, whereby the thermal conductivity of the resistor element and consequently the output characteristics of the element are changed. As a result, there arises a deviation of the output of the resistor element from the actual flow rate of the intake air. This deviation gradually increases with an increase in the amount of the contaminants attached to the resistor element, with a result of chronological reduction in the detecting accuracy of the flowmeter.
In view of the above, it has been proposed to provide a protective member or members of various mesh structures, which is/are located upstream and/or downstream of the heat-generating resistor element disposed in the intake air passage, so as to capture dust, water, oil and other contaminants which would otherwise be attached to the resistor element.
Some examples of such protective member are disclosed in JP-A-54-74779, which have various kinds of mesh structures such as a sieve mesh, honeycomb mesh, and a mesh consisting of a bundle of tubes. Another form of protective member is disclosed in JP-A-63-243720, which is a network or honeycomb metal structure having good electric conductivity, adapted for removing an electrostatic charge from dust which passes through the structure. It is also proposed in JP-A-1-169323 to heat a mesh-like member for capturing dust in the intake air, to thereby enhance its capability of capturing the dust. A further form of protective member as disclosed in JP-U-1-102722 is a net which is located in the vicinity of a heat-generating resistor element on the upstream side thereof as viewed in a direction in which the intake air flows. This net is adapted to cause a slight turbulence of the air passing therethrough, so as to give a rotating force to dust passing through the net and thereby prevent the dust from attaching to the resistor element.
However, the protective members as described above are all formed from a metal sheet, a metal wire and the like having a relatively smooth surface, to which dust, water, oil and other contaminants are less likely to be attached. Thus, the known protective members suffer from low capability of capturing the contaminants. Further, the dust, water, oil and others once attached to the protective member are likely to be removed from the member, and eventually pass through the protective member and are attached to the heat-generating resistor element. Thus, the amount of the contaminants attached to the resistor element cannot be effectively reduced by the known protective members.